1. Field of the Invention
The present invention relates to an ultrasonic transducer for transmitting and receiving ultrasonic waves, and a method for manufacturing the ultrasonic transducer, and an ultrasonic flowmeter using the ultrasonic transducer.
2. Description of Prior Art
In recent years, an ultrasonic flowmeter which measures the time of flight of an ultrasonic wave across the propagation path, and determines the passing speed of a fluid, thereby to measure the flow rate has come into use for a gas meter or the like.
FIG. 1 is a diagram showing the measurement principle of an ultrasonic flowmeter. As shown in FIG. 1, a fluid flows in the direction indicated by an arrow at a velocity V in a tube. A pair of ultrasonic transducers 101 and 102 are oppositely mounted on a tube wall 103. The ultrasonic transducers 101 and 102 are respectively configured with piezoelectric vibrators such as piezoelectric ceramics as electrical energy/mechanical energy conversion elements. Herein, the ultrasonic transducer 101 is used as an ultrasonic transmitter, and the ultrasonic transducer 102 is used as an ultrasonic receiver.
The operation is as follows. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 101 to the piezoelectric vibrator, the ultrasonic transducer 101 emits an ultrasonic wave into an external fluid along a propagation path denoted by L1 in the diagram. Then, the ultrasonic transducer 102 receives the propagated ultrasonic wave, and converts it into a voltage. Subsequently, the ultrasonic transducer 102 is used as an ultrasonic transmitter, and the ultrasonic transducer 101 is used as an ultrasonic receiver. Upon application of an alternating voltage with a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 102 to the piezoelectric vibrator, the ultrasonic transducer 102 emits an ultrasonic wave into the external fluid along a propagation path denoted by L2 in the diagram. Then, the ultrasonic transducer 101 receives the propagated ultrasonic wave, and converts it into a voltage.
Further, with such an ultrasonic transducer, if an alternating voltage is successively applied thereto, ultrasonic waves are successively emitted from the ultrasonic transducer. Accordingly, it becomes difficult to determine the time of flight. For this reason, in general, a burst voltage signal using a pulse signal as a carrier wave is used as a driving voltage. Hereinafter, the measurement principle will be described in more details. Upon application of a burst voltage signal for driving to the ultrasonic transducer 101, an ultrasonic pulse wave is emitted from the ultrasonic transducer 101. The ultrasonic pulse wave propagates through the propagation path L1 with a length L, and reaches the ultrasonic transducer 102 after (time of flight) t hours. With the ultrasonic transducer 102, the propagated ultrasonic pulse wave can be converted into an electrical pulse wave at a high S/N ratio. By using the electrical pulse wave as a trigger signal, the ultrasonic transducer 101 is driven again to emit an ultrasonic pulse wave. This device is referred to as a sing-around device. The time required for an ultrasonic pulse to be emitted from the ultrasonic transducer 101, and propagate through the propagation path to reach the ultrasonic transducer 102 is referred to as a sing-around period. The inverse thereof is referred to as a sing-around frequency.
In FIG. 1, a reference character V denotes the flow velocity of the fluid flowing in the pipe, C denotes the velocity of an ultrasonic wave in the fluid, and θ denotes the angle between the direction of flow of the fluid and the direction of propagation of an ultrasonic pulse. When the ultrasonic transducer 101 is used as a transmitter, and the ultrasonic transducer 102 is used as a receiver, the following equation (1) holds:f1=1/t1=(C+V cos θ)/L   (1)
where t1 denotes the sing-around period which is the time for an ultrasonic pulse emitted from the ultrasonic transducer 101 to reach the ultrasonic transducer 102, and f1 denotes the sing-around frequency.
In contrast, when the ultrasonic transducer 102 is used as a transmitter, and the ultrasonic transducer 101 is used as a receiver, the following equation (2) holds:f2=1/t2=(C−V cos θ)/L   (2)
where t2 denotes the sing-around period therefor, and f2 denotes the sing-around frequency.
Accordingly, the frequency difference Δf between both the sing-around frequencies is expressed as the following equation (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf:Δf=f1−f2=2 V cos θ/L   (3)
Namely, it is possible to determine the flow velocity V of the fluid from the length L of the propagation path for the ultrasonic wave, and the frequency difference Δf. Therefore, it is possible to determine the flow rate from the flow velocity V.
Such an ultrasonic flowmeter is required to have a high degree of precision. In order to improve the precision, the acoustic impedance of a matching layer becomes important which is formed on the transmitting and receiving surface of ultrasonic waves in the piezoelectric vibrator configuring the ultrasonic transducer for transmitting ultrasonic waves to a gas, or receiving the ultrasonic waves propagated through the gas. The acoustic impedance of the piezoelectric vibrator for generating the ultrasonic vibrations is about 30×106. The acoustic impedance of air is about 400. The ideal value of the acoustic impedance of the acoustic matching layer is about 0.11×106. Further, the acoustic impedance is defined as the following equation (4):Acoustic impedance=(density)×(sound velocity)
Therefore, a low density material, such as a material obtained by solidifying a glass balloon or a plastic balloon with a resin material, is used for the acoustic matching layer for controlling the acoustic impedance at a low level. Alternatively, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like. The method is disclosed in Japanese Patent Publication No. 2559144, or the like.
For the acoustic matching layer used in a conventional ultrasonic transducer used for an ultrasonic flowmeter, there has been adopted a method in which a hollow glass ball is thermally compressed, a molten material is foamed, or the like, as described above. For this reason, there occur the following problems. The medium tends to be heterogeneous due to fracture of the glass ball under pressure, separation under insufficient pressure, foaming of the peeled molten material, or the like. Accordingly, variations occur in characteristics, which then generates variations in device precision. Further, there also occur the following problems. For example, since the acoustic matching layer is exposed to a gas, the surface is collapsed by the moisture, or the layer is easily deteriorated by a chemically active substance, resulting in inferior durability.